Interpretive Summary: Infection with the single-celled organism Toxoplasma gondii is common in man and animals. It causes mental retardation and loss of vision in congenitally infected children and abortion in livestock. There is no vaccine to prevent T. gondii infection in humans or animals. Scientists at the Beltsville Agricultural Research Service and the National Institute of Health report the results of a preliminary trial on the effectiveness of a genetically engineered vaccine using T. gondii DNA to reduce toxoplasmosis in rodents. These results will be useful to parasitologists, veterinarians and physicians.

Technical Abstract:
Immunization with DNA can induce humoral and cell-mediated immune responses, both of which are important in conferring immunity to T gondii. We evaluated the immunogenicity and efficacy of genetic vaccination with a cDNA construct encoding the T. gondii SAG1 (P30) surface antigen. Sera of immunized mice were positive against T. gondii tachyzoites by immunofluorescence and exhibited high titers against SAG1 by ELISA. SAG1- stimulated splenocytes from vaccinated mice produced primarily IFN-y and IL-2, with little production of IL-4. All vaccinated mice survived challenge with 80 T. gondii tissue cysts, while all control mice died; challenge with 20 cysts resulted in significantly fewer brain cysts as compared to controls. Challenge of vaccinated rats with VEG strain resulted in a significant reduction of brain cysts. These results suggest that nucleic acid vaccination can provide protection against T. gondii infection.